1. Field of the Invention
The present invention relates to an error diffusion processing circuit for an image processor such as a monochrome or color laser printer and a monochrome or digital color copier.
2. Description of the Prior Art
In prior art an image output/input system in which multivalued image data are read with an input device as a scanner or a digital camera and output with an output device as a printer or a display is well known. In such a system the read multivalued (for example, 8-bit, 256 tones) data is subjected to pseudo tone processing using error diffusion to convert, using a predetermined quantization threshold, the data into quantized image data in the number of tones with which the output device can process. Quantization errors occur in the conversion of the original multivalued image data into quantized image data. The error diffusion is to diffuse the quantization errors into surrounding pixels and thereby reduce errors in the entire quantized image data. The use of dithering threshold matrices as the quantization thresholds has been proposed.
For example, Japanese Patent Publication No. 3963260 discloses an image processor which includes a threshold generator having threshold groups for determining different density areas to generate each threshold for multi-toned image data of one pixel, and a tone processor to convert the multi-toned image data into less toned quantized data by the error diffusion using the thresholds. The threshold groups correspond to the quantization thresholds above.
Moreover, Japanese Patent Application Publication No. 2005-260399 discloses an image processor which extracts the characteristic amount of image data tone-converted referring to a tone conversion table and changes a quantization threshold for use in the error diffusion on the basis of the extracted characteristic amount.
The image processors above include a memory in which a tone conversion table for each characteristic amount is pre-stored and selectively change the tone conversion table according to the characteristic amount of an image. Therefore, depending on subject image data, all of the tone conversion tables are not always used and the use of the tables is not efficient. Meanwhile, to increase the number of tone conversion tables, the size of the memory needs to be increased proportionally.